Two-dimensional (2D) or three-dimensional (3D) magnetic field sensors give rise to a strong interest for vast fields of application such as the automobile, avionics or space industry, mobile or on-board systems, mobile telephony, the field of personal computers, petrochemistry, the environment or biomedical, etc. New solutions are sought in order to obtain 2D or 3D magnetic field sensors with the following properties:                high sensitivity according to each dimension,        low energy consumption,        low weight, high degree of lightness,        miniaturisation for minimum encumbrance,        facility for collective manufacturing for mass production at relatively low costs,        easy to use.        
Indeed, the existing 2D or 3D magnetic field sensors have limitations concerning their performance or their manufacturing techniques.
A 2D or 3D magnetic field sensor allows for the directional measurement of the magnetic field B by the measurement, simultaneously and without having to reorient the sensor in space, of two or three components of the magnetic field B. A 3D magnetic field sensor makes it possible as such, by the simultaneous measurement of three components (Bx, By, Bz) of the magnetic field B in space, to carry out a mapping of the magnetic field vector in the region explored, i.e. in a given field range: this can be a mapping of the variations of the magnetic field vector in space, or a mapping of the variations of the magnetic field vector over time, possibly at high frequency. It is typically desired that a 2D or 3D magnetic field sensor have the same measurement sensitivity over its various dimensions and be as sensitive, over each one of its dimensions, as a uniaxial magnetic field sensor. The existing uniaxial magnetic field sensors are for example sensors with giant magnetoresistance GMR, sensors with anisotropic magnetoresistance AMR, sensors with tunnel magnetoresistance TMR or microfluxgate sensors. Such sensors are sometimes called “vector sensors” as they measure a vector component of the magnetic field applied.
According to the cases, the ambient magnetic fields to be measured can come from local sources of currents or from magnetic materials such as magnets. It can also concern measuring the terrestrial or spatial magnetic field, or measuring fields created by living organisms. Generally, the magnetic fields to be measured typically have weak amplitudes and/or weak variations that are desired to be detected. The desired measurement ranges can be highly varied. A strong field to be measured can sometimes be higher than Tesla T. More commonly, the fields to be measured are of a magnitude of the millitesla mT, of the microtesla pT (as such the local terrestrial field is of a magnitude of 50 μT environ) or of the nanotesla nT. The fields to be measured can also be of a magnitude of the picotesla pT or of the femtotesla fT, etc. An objective can for example be to measure in three dimensions weak variations in the terrestrial field, for example of the magnitude of the nT, or very weak values of the magnetic field in space. Certain fields such as space, geomagnetometry or biomagnetometry in particular require magnetic field sensors that have high performance in terms of sensitivity.
Uniaxial magnetic sensors, or monoaxial magnetic sensors, such as GMR sensors, are known for measuring the amplitude of the magnetic field on the axis of the sensor. Uniaxial magnetic sensors are typically carried out on the surface of a substrate by microelectronic techniques.
FIG. 1 shows by way of example a uniaxial sensor GMR 1 according to prior art. Such a uniaxial sensor GMR is also called a “spin valve”.
The uniaxial sensor GMR 1 comprises a flux concentrator CF that comprises a first magnetic part PM1 and a second magnetic part PM2. The first magnetic part PM1 and the second magnetic part PM2 are separated by an air gap E. In the example of FIG. 1, the flux concentrator F has a dimension L1, referred to as “large dimension”, according to an X axis and a dimension L2, referred to as “small dimension”, according to a Y axis. The Y axis is perpendicular to the X axis. The flux concentrator F makes it possible to capture the magnetic flux that is created by the field B to be measured in the direction X of its large dimension L1, and to amplify this field B to be measured on a magnetoresistive element MR. The magnetoresistive element MR is typically inserted into the air gap E of the flux concentrator F. The first and second magnetic parts PM1 and PM2 of the flux concentrator F are made from a soft material, which is typically permalloy.
The magnetoresistive element MR of the uniaxial sensor GMR 1 makes it possible to transform a variation in the magnetic field into a variation of electrical resistance, which is measured thanks to two electrical contacts ct1 and ct2. In the case of the uniaxial sensor GMR 1, the magnetoresistive element MR is a spin valve that uses the phenomenon of giant magnetoresistance. Alternatively, other types of uniaxial sensors can use a magnetoresistive element which is a magnetic tunnel junction. A spin valve is in particular constituted of a magnetic layer of which the magnetisation direction Mref is trapped, called “reference layer”, and of a magnetic layer of which the magnetisation direction Mvar is variable, called “soft layer”. The magnetisation direction Mvar of the soft layer easily turns under the influence of a field B to be measured. The reference layer and the soft layer are separated by a non-magnetic layer which is typically made of copper. The trapping of the reference layer is in general carried out by interaction with an antiferromagnetic layer adjacent to the reference layer, by an anisotropic exchange mechanism. The antiferromagnetic layer is not shown in FIG. 1. Alternatively, the reference layer can be a synthetic antiferromagnetic layer comprising two ferromagnetic layers coupled in an antiparallel manner through a fine layer of antiparallel coupling often made of ruthenium with a thickness less than 1 nm. This synthetic antiferromagnetic layer can itself be trapped by interaction with another antiferromagnetic layer. In the example of FIG. 1, the magnetisation direction Mref of the reference layer is parallel to the field B to be measured, i.e. parallel to the X axis, and in a zero field, the magnetisation direction Mvar of the soft layer is transverse to the field B to be measured, i.e. parallel to the Y axis. In the case of the uniaxial sensor GMR 1 using a spin valve, the electrical contacts ct1 and ct2 are arranged on either side of the magnetoresistive element MR, i.e. on either side of the spin valve, in such a way that an electric current can flow in the plane of the layers: this is a known CIP (“Current In Plane”) configuration. In the alternative case of a uniaxial sensor that uses a magnetic tunnel junction, the electrical contacts ct1 and ct2 are taken one under the stack of the layers and the other on the stack of the layers in such a way that an electric current can pass through the magnetic tunnel junction perpendicularly to the tunnel barrier.
The magnetic field radiated in the air gap E that separates the two magnetic parts of the flux concentrator F is very weak when the field applied is zero. On the other hand, when the field is applied according to the large dimension L1 of the flux concentrator CF, the soft material of the magnetic parts PM1 and PM2 is polarised magnetically parallel to the field and creates a strong field radiated in the air gap E of the flux concentrator CF wherein is arranged the magnetoresistive element MR. Under the effect of the field B to be measured, the magnetisation direction Mvar of the soft layer of the magnetoresistive element MR turns and approaches either the parallel alignment, or the antiparallel alignment in relation to the magnetisation direction Mref of the reference layer. When the magnetoresistive element MR is passed through by an electric current, this results in a voltage at the terminals of the magnetoresistive element MR which varies according to the field B applied on the magnetoresistive element MR. In the case of the uniaxial sensor GMR 1, the resistance of the spin valve varies as the cosine of the angle between the magnetisation direction Mref of the reference layer and the magnetisation direction Mvar of the soft layer. In the alternative case of a uniaxial sensor that uses a magnetic tunnel junction, this is the conductance of the magnetic tunnel junction that varies as the cosine of the angle between the magnetisation direction Mref of the reference layer and the magnetisation direction Mvar of the soft layer.
Still in the case of a uniaxial measurement, as the measurement direction is defined by the large dimension L1 of the flux concentrator, a first permanent magnet A1 and a second permanent magnet A2 are arranged on either side of the magnetoresistive element MR. The first and second permanent magnets A1 and A2 are typically made from a hard magnetic material and create a field Hbias that polarises the magnetisation direction Mvar of the soft layer transversally to the direction of the field to be measured. It is as such ensured that the magnetisation direction Mvar of the soft layer in zero field is single-domain and polarised transversally to the direction of the field to be measured. The transverse field Hbias is not excessively large in order to not excessively reduce the sensitivity of the sensor. At the ends of the magnetoresistive element MR in contact with each permanent magnet, the transverse field Hbias is typically of a magnitude of 0.1 to 1 times the maximum value of the field that is sought to be measured. The transverse field Hbias is lower at the centre of the magnetoresistive element MR than at its ends, due to a greater separation with respect to the permanent magnets. The transverse field Hbias makes it possible to substantially reduce the noise of the sensor by preventing the formation of complex magnetic structures in the sensitive layer.
A 2D magnetic field sensor can typically be obtained by placing on the surface of a substrate a first uniaxial sensor that has a first measurement axis and a second uniaxial sensor having a second measurement axis, with the second measurement axis forming a non-zero angle, generally 90°, with the first measurement axis. Such a realisation with two different orientations of magnetic sensors is obtained by microelectronic methods. However, manufacturing a 2D magnetic field sensor of the GMR or TMR type by association of two monoaxial sensors orthogonal to the surface of a substrate imposes in the state of the art a certain technological complexity.
FIG. 2 shows by way of example a 2D magnetic field sensor according to prior art, comprising a first uniaxial sensor 1-x of measurement axis X and a second uniaxial sensor 1-y of measurement axis Y. The first sensor 1-x of measurement axis X makes it possible to measure a first component B-x of a magnetic field applied and the second sensor 1-y of measurement axis Y makes it possible to measure a second component B-y of the magnetic field applied. In a manner similar to what has been described hereinabove, the first sensor 1-x comprises;                a flux concentrator CF-x comprising a first magnetic part PM1-x, a second magnetic part PM2-x and an air gap E-x that separates the first and second magnetic parts;        a magnetoresistive element MR-x comprising a reference layer of which the magnetisation direction Mref-x is fixed according to the X axis and a soft layer of which the magnetisation direction Mvar-x is variable;        a first permanent magnet A1-x and a second permanent magnet A2-x that create a transverse field Hbias-x that polarises the magnetisation direction Mvar-x of the soft layer transversally to the magnetisation direction Mref-x of the reference layer, i.e. according to the Y axis;        a first electrical contact ct1-x and a second electrical contact ct2-x.         
The second sensor 1-y comprises:                flux concentrator CF-y comprising a first magnetic part PM1-y, a second magnetic part PM2-y and an air gap E-y that separates the first and second magnetic parts;        a magnetoresistive element MR-y comprising a reference layer of which the magnetisation direction Mref-y is fixed according to the Y axis and a soft layer of which the magnetisation direction Mvar-y is variable;        a first permanent magnet A1-y and a second permanent magnet A2-y that create a transverse field Hbias-y that polarises the magnetisation direction Mvar-y of the soft layer transversally to the magnetisation direction Mref-y of the reference layer, i.e. according to the X axis;        a first electrical contact ct1-y and a second electrical contact ct2-y.         
The fact that the measurement axis of the magnetoresistive element MR-x of the first sensor 1-x forms an angle, typically 90°, with the measurement axis of the magnetoresistive element MR-y of the second sensor 1-y imposes to trap in a different manner the magnetisation direction Mref-x of the reference layer of the first sensor 1-x on the one hand, and the magnetisation direction Mref-y of the reference layer of the second sensor 1-y on the other hand. This also imposes to polarise differently the magnetisation direction Mvar-x of the soft layer of the first sensor 1-x on the one hand, and the magnetisation direction Mvar-y of the soft layer of the second sensor 1-y on the other hand. In the example of FIG. 2, this therefore relates to trapping the magnetisation direction Mref-x of the reference layer of the first sensor 1-x according to the X axis, and to trapping the magnetisation direction Mref-y of the reference layer of the second sensor 1-y according to the Y axis. Still in the example of FIG. 2, this entails polarising the magnetisation direction Mvar-x of the soft layer of the first sensor 1-x according to the Y axis, and polarising the magnetisation direction Mvar-y of the soft layer of the second sensor 1-y according to the X axis.
However this double constraint—trapping differently the magnetisation directions of the reference layers and polarising differently the magnetisation directions of the soft layers—substantially increases the complexity the technology of modifying such a 2D magnetic field sensor.
Indeed, the trapping of the magnetisation direction of a reference layer of a spin valve is carried out typically via an annealing and a cooling under the field of the spin valve from the blocking temperature of the antiferromagnetic trapping layer of the reference layer, Orienting the magnetisation directions of a first reference layer and of a second reference layer according to two different directions therefore requires that the blocking temperature of the antiferromagnetic trapping layer of the first reference layer be different from the blocking temperature of the antiferromagnetic trapping layer of the second reference layer. The first reference layer can for example have an IrMn antiferromagnetic layer, while the second reference layer has a PtMn antiferromagnetic layer. But this implies that the two uniaxial sensors of the 2D magnetic field sensor cannot be manufactured in a single technological step, and must on the contrary be manufactured in technologically different steps. The manufacturing is therefore rendered more complex and the manufacturing costs are increased. Another possibility is to apply different local fields on the two uniaxial sensors during the annealing. This cannot be done with macroscopic magnets and typically requires the adding of an elbowed conductor line passing over the two uniaxial sensors and wherein a current is made to flow during the annealing and the cooling. This current generates a field in two orthogonal directions if the line has an elbow that is suitably arranged in relation to the position of the two uniaxial sensors. However, the carrying out of this elbowed conductor line also renders the manufacturing technology more complex.
Moreover, the permanent magnets used to polarise the magnetisation directions of the soft layers are made of hard magnetic materials, of the alloy type with a base of Co and of Cr, or of Sm and of Co, or of NdFeB in thin layers. The orientation of the magnetisation directions of these permanent magnets is carried out by applying a strong magnetic field, i.e. above the coercive field of the material, which induces a remanent magnetisation in the desired direction. This strong magnetic field should however be applied over the entire wafer, in that there is no simple solution for applying such a strong magnetic field on a local scale. In this case, the magnetisation directions of the permanent magnets are parallel, which is not satisfactory. Alternatively, certain magnetic field sensors do not use permanent magnets and use non-polarised soft layers, or polarised soft layers by weakly coupling their magnetisation direction with an antiferromagnetic layer. In this case, the reference layer and the soft layer are both coupled to an antiferromagnetic layer: the reference layer is strongly coupled to a first antiferromagnetic layer in order to block its magnetisation direction, while the soft layer is weakly coupled to a second antiferromagnetic layer, in such a way that the second antiferromagnetic layer exerts a weak polarisation field on the magnetisation direction of the soft layer, but that the magnetisation direction of the soft layer remains variable and can still turn under the effect of a magnetic field applied. In this configuration, for a given uniaxial sensor, in order to orient the magnetisation direction of the reference layer orthogonally to the direction of polarisation of the soft layer, a first antiferromagnetic layer is used, having a blocking temperature that is different from the blocking temperature of the second antiferromagnetic layer. The antiferromagnetic layer that has the highest blocking temperature is oriented first, then the field applied is turned 90° before orienting the antiferromagnetic layer that has the lowest blocking temperature. However, there is no simple solution afterwards for initialising the two uniaxial sensors in relation to one another.
The difficulties linked to manufacturing a 2D magnetic field sensor, allowing for the measurement of a first magnetic field component Bx and of a second magnetic field component By in the plane of a substrate, have been described hereinabove. If it is now sought to manufacture a 3D magnetic field sensor, new difficulties arise. Indeed, the measurement of a third magnetic field component Bz outside the plane of the substrate is much more complex to obtain simultaneously and with the same precision as the first and second magnetic field components Bx and By in the plane of the substrate. In order to measure the third component with the same sensitivity as the first and second components, this entails using a third uniaxial sensor outside of the surface of the substrate, the third uniaxial sensor being of the same type as the first and second uniaxial sensors on the surface of the substrate. The third uniaxial sensor is perpendicular to the surface of the substrate. But from a technological standpoint, there is no simple solution that makes it possible to obtain such a third uniaxial sensor having a high sensitivity with respect to the third magnetic field component Bz. In general, the third uniaxial sensor is carried out on a plane that is inclined in relation to the plane of the substrate, or is carried out separately then fixed perpendicularly to the plane of the first and second uniaxial sensors. In both cases, the method of manufacture of the 3D magnetic field sensor is rendered complex.